The present invention relates to electronic devices, and, more particularly, to semiconductor devices useful with memory devices.
Computers frequently include a timekeeping function that keeps track of the time of day, day, month, and year for various uses. Such timekeeping function will typically be implemented in specific hardware with a dedicated crystal oscillator to insure accuracy and a battery backup power supply to insure preservation of timekeeping data during an interruption of the primary power supply. This is especially important with personal computers which are not infrequently powered down.
In addition to timekeeping data, computers often also contain data in main memory that preferably would not be lost upon power interruption. Thus devices such as the DS1287 Real Time Clock manufactured by Dallas Semiconductor Corp. provide a single chip that includes both a timekeeping function with battery backup and crystal oscillator plus 50 bytes of nonvolatile general purpose RAM. The DS1287 is designed to fit into an IBM AT type personal computer and other such machines. Of course, larger nonvolatile RAM would be more desirable, and such a timekeeping plus RAM circuit must be very low power to provide long battery lifetime. However, merely attaching an external RAM to a device such as the DS1287 gives rise to a problem of complicated memory mapping which detracts from the simplicity of attachment of external RAM.
The present invention provides a clock with on chip RAM plus address control for the direct attachment of external RAM with continguous address mapping.